Switching Devices. Micromechanical devices (sometimes known as MEMS devices) have been known for many years, and various switch designs have been proposed using MEMS technology. However, the designs presently available still have shortcomings. For example, none has proven suitable for switching high power radio frequency signals (e.g., 5 W of RF power at 0.1-6 GHz). It is generally considered essential to obtain a large contact force for reliable high-power switches, and this can only be done currently using thermal actuation. Cronos (later JDS Uniphase) developed a thermal actuation switch beginning in 1999 with low insertion loss and high isolation at 0.1-6 GHz [RF MEMS: Theory, Design and Technology, John Wiley and Sons, February 2003; R. Wood, R. Mahadevan, V. Dhuler, B. Dudley, A. Cowen, E. Hill, and K. Markus, MEMS microrelays, Mechatronics, Vol. 8, pp. 535-547, 1998]. This switch resulted in about 1 mN of contact force per contact, used a pure gold contact, and was tested up to 25 W for 50 million cycles in a tunable 50 MHz filter by the Raytheon group with no failures [R. D. Streeter, C. A. Hall, R. Wood, and R. Madadevan, VHF highpower tunable RF bandpass filter using microelectromechanical (MEM) microrelays, Int. J. RF Microwave CAE, Vol. 11, No. 5, pp. 261-275, 2001; Charles A. Hall, R. Carl Luetzelschwab, Robert D. Streeter, and John H. VanPatten, “A 25 Watt RF MEM-tuned VHF Bandpass Filter,” IEEE Int. Microwave Symp., pp. 503-506, June 2003]. However, the switch consumed 250 mW of continuous DC power for operation, and the tunable filter with 8 actuated switches on average required 2 Watts of DC control power. The University of California, Davis, improved the Cronos design by using a more efficient thermal actuator and dropped the drive power from 250 mW to 60-70 mW for a 0.5 mN of contact force [Y. Wang, Z. Li, D. T. McCormick, and N. C. Tien, Low-voltage lateral-contact microrelays for RF applications, in 15th IEEE International Conference on Micro-Electro-Mechanical Systems, January 2002, pp. 645-648]. While an improvement over the previous design, this was still not acceptable for phased arrays and complicated switch networks. The Cronos switch was not used by the DoD or commercial community due to its high control power, but it demonstrated that acceptable switch performance can be obtained with 1-2 mN of contact force per contact.
Some designs reduce the required control power with a latching switch. In a latching switch, the control power is activated for only 0.3-3 milliseconds. This can be suitable for slow scanning phased arrays on unmanned air vehicles or in satellite systems. A latching switch also keeps its state if the power is temporarily lost (or purposely removed), which can be a great advantage in set-and-forget systems such as large switch networks for automated testing of defense and commercial systems, or in satellite applications with large pipe-line switch networks. A principal component of many latching switch designs is a bi-stable spring and actuation mechanism. A switch by Magfusion (formerly Microlab) is rated to 10 mA only for 10 million cycles [RF MEMS: Theory, Design and Technology, John Wiley and Sons, February 2003, M. Ruan, J. Shen, and C. B. Wheeler, Latching Micromagnetic Relays, IEEE J. Microelectromech. Systems, Vol. 10, pp. 511-517, December 2001. Also, see www.magfusion.com] since it has low contact forces, of the order of 0.1 mN and uses a gold contact. Thermal latching switches switches by Michigan (and MIT) have not yet seen commercial acceptance [Long Que, Kabir Udeshi, Jaehyun Park, and Yogesh B. Gianchandani, “A BI-STABLE ELECTRO-THERMAL RF SWITCH FOR HIGH POWER APPLICATIONS,” IEEE Conf. on Micro-electro-mechanical Systems, pp. 797-800, January 2004; J. Qiu, J. H. Lang, A. H. Slocum, R. Strümpler, “A High-Current Electrothermal Bistable MEMS Relay,” MEMS'03, pp. 64-67, 2003]. Latching-type switches are generally quite large due to the bi-stable spring used, and therefore are not generally suited for high microwave or mm-wave operation.
Another set of RF MEMS switches include the Radant MEMS metal-contact switch with electrostatic actuation [S. Majumder, J. Lampen, R. Morrison and J. Maciel, “A Packaged, High-Lifetime Ohmic MEMS RF Switch,” IEEE MTT-S Int. Microwave Symp., pp. 1935-1938, June 2003], and the Raytheon capacitive switch [RF MEMS: Theory, Design and Technology, John Wiley and Sons, February 2003], also with electrostatic actuation. Both are very small, have been taken to mm-wave frequencies, and have been tested for at least 20 Billion cycles and in some cases to 100 Billion cycles. However, the Radant switch results in 0.1 mN of contact forces and cannot handle 5 W of RF power, and the Raytheon capacitive switch is not suitable for 0.1-6 GHz applications.
Current switch designs suffer from various shortcomings, which have so far precluded development of a high-power latching RF MEMS switch.
Safing Devices. In order to prevent an energetic material used in a rocket motor, warhead, explosive separation device or other similar device, collectively sometimes referred to as “target devices”, from being unintentionally operated during handling, flight or in any circumstance that could produce an extreme hazard to personnel or facilities, a “safing device” is customarily incorporated in the firing control circuit for the foregoing devices as a safety measure. These generically fall into two categories: “arm/fire” and “safe and arm”. The arm/fire device electrically and/or mechanically interrupts the “ignition train” to the target device so as to prevent accidental operation. The arm/fire device includes a mechanism that permits the target device to be armed, ready to fire, only while electrical power is being applied to the target device. When that electrical power is removed, signifying the target device is disarmed, the mechanism of the arm/fire device returns to a safe position, interrupting the path of the ignition train.
The safe and arm device is of similar purpose, and is a variation of the arm/fire device. The mechanism of the safe and arm device enables the target device, such as the rocket motor, warhead and the like, earlier mentioned, to remain armed, even after electrical power is removed. The device may be returned to a “safe” position only by applying (or reapplying) electrical power. The safe and arm device is commonly used to initiate a system destruct in the event of a test failure, for launch vehicle separation and for rocket motor stage separation during flight. Typically, the safe and arm device uses a pyrotechnic output which may be either a subsonic pressure wave or which may be a flame front and supersonic shock wave or detonation to transfer energy to another pyrotechnic device (and serves as the trigger of the latter device).
Existing safety devices are typically of the size of a person's fist, and possess a noticeable weight of several pounds. Although MEMS and other microfabrication technologies have been brought to bear on such safing devices, it has been primarily in the area of the ignition device that initiates the ignition train or in only a portion of the mechanism. There are currently no completely microfabricated safing devices available. Microfabrication of a safing device can allow significant reduction of weight, volume and cost. Reduction of weight and volume of those devices can allow corresponding increases in weight and/or volume of payload and propulsion systems resulting in increased range and capability of a weapon system. Reduced size and cost can allow the safing of small munitions or sub-munitions that are currently not provided with safing systems.